1. Field of the Invention
The present invention relates to a control device for an internal combustion engine provided with a three-way catalyst for exhaust gas purification in an exhaust system, and more specifically to a control device for an internal combustion engine having a failure determining function that carries out a catalyst failure determination on the basis of results of the detection of air-fuel ratios on the upstream and the downstream side of the catalyst.
2. Description of the Related Art
For example, an exhaust system of an internal combustion engine for a vehicle is provided with a three-way catalyst for exhaust gas purification. Feedback control is performed to bring exhaust gas close to a theoretical air-fuel ratio, and the three-way catalyst then carries out the oxidation of hydrocarbon (HC) and carbon monoxide (CO) and the reduction of nitrogen oxide (NOx), thereby decreasing harmful substances contained in the exhaust gas. A three-way catalyst of this type is gradually deteriorated (fails) in the course of being used, and is reduced in purification efficiency. In order to prevent the emission of harmful substances into the atmosphere when a catalyst that has failed continues to be used, by way of example, legislative regulations for OBD (on-board diagnosis) in North America stipulates that a failure determining function that detects and indicates a catalyst failure to prompt repair be provided to the vehicle.
Accordingly, a failure determining device that makes a catalyst failure determination on the basis of output of O2 sensors installed upstream and downstream of the catalyst has been employed. The failure determining device operates through the use of the phenomenon that the feedback control causes the air-fuel ratio of exhaust gas to fluctuate at short periods around the vicinity of the theoretical air-fuel ratio, and the fluctuation of the air-fuel ratio is suppressed downstream from the catalyst because of the oxygen storage ability of the catalyst. In other words, so long as the catalyst retains the sufficient purification ability, the amplitude of fluctuation of the air-fuel ratio grows relatively small on the downstream side of the catalyst because of the oxygen storage ability, and at the same time a reverse frequency of between rich and lean conditions becomes relatively low. If the oxygen storage ability is declined as the catalyst deteriorates, this increases the amplitude of fluctuation of the air-fuel ratio on the downstream side of the catalyst, and also increases the reverse frequency, compared to the case in which the catalyst retains the sufficient purification ability. For this reason, the failure determining device makes a failure determination at the point when the reverse frequency ratio (the reverse frequency of the downstream O2 sensor/the reverse frequency of the upstream O2 sensor≦1.0) exceeds the specified failure determination value.
One of the measures for the suppression of discharge of harmful substances in the event of a catalyst failure is proposed, for example, in Unexamined Japanese Patent Publication No. 2001-263100 (hereinafter referred to as Patent Document 1). According to the technology disclosed in Patent Document 1, when a catalyst failure determination is made, for example, through the above-mentioned technique or the like, a variable valve timing mechanism is operated to advance the closing timing of the exhaust valve of an internal combustion engine and retard the opening timing of the intake valve, to thereby cause exhaust gas to remain in cylinders to lower NOx.
The technology disclosed in Patent Document 1, however, is employed only after the catalyst actually fails, and no measures are taken before the catalyst failure. As a consequence, the catalyst deterioration causes the problem that harmful substances are discharged into the atmosphere.
Furthermore, it is hard to say that the technique for making a catalyst failure determination can cope satisfactorily with the strengthening of the regulations especially with regard to NOx in late years. FIG. 6 shows the relationship of an O2 sensor reverse frequency ratio with respect to NOx emissions from the catalyst in the process of catalyst deterioration. In FIG. 6, for example, if a failure determination value for making a comparison to the reverse frequency ratio in the event of a failure determination is set to be 0.8, the failure determination is made at point a of an output characteristic shown by a solid line. Therefore, the failure determination value can comply with the current OBD regulation value. On the other hand, however, it is impossible to make a failure determination before the failure determination value exceeds the future OBD regulation value that is lower than the current one.
One of possible measures against the above difficulty is, for example, to reduce the failure determination value to 0.4 and make a failure determination at point b before the NOx emissions exceed the future OBD regulation value. In this case, however, the accuracy of the failure determination is decreased, leading to an erroneous determination. Moreover, even if no erroneous determination is made, the NOx emissions exceed the regulation value in spite of the fact that the catalyst is perfectly capable of HC purification, which produces another problem that the frequent replacement of an expensive three-way catalyst made of precious metals is required.